Ethylene copolymer molecular weight distribution and related parameters are an important predictor of polymer processability. Generally, an increase in the molecular weight distribution means an increase in polymer processability. However, changes in polymer molecular weight distribution may also affect polymer properties such as the optical properties and toughness. Thus, while processability is an important parameter in, for example, film extrusion applications, as the molecular weight distribution, stress exponent, and melt flow ratio of a polyethylene increase, they can have a negative impact on blown film properties. For example, polymer optical properties, tear strengths as well as dart impact properties can be negatively impacted. Also, the processability and physical property requirements of a polyethylene may be different for different polymer uses and applications.
In the gas phase polymerization of ethylene (optionally with comonomers), a fluidized bed reactor system is often employed. Generally these reactor systems include a reactor having two reaction zones, and a recycle loop which comprises a compressor and a heat exchanger. A two phase system is maintained in the reaction zone where growing particles of polyethylene resin are a solid suspended (i.e. fluidized) within a gaseous flow entering the reactor through a distributor plate. Catalyst is injected via an injection line, and the heat exchanger removes the heat of the polymerization reaction.
Generally speaking, a goal when making polyethylene products in a fluidized bed using a given catalyst system is to have a reactor system that results in products with uniform properties which are consistently and reliably made even where polymerization conditions (such as comonomer feed, reaction temperature, condensable hydrocarbon concentration etc.) may be subject to minor process fluctuations. Indeed, robust catalyst systems are able to make a specific product reliably and without too much sensitivity to changes in process conditions. However, there is a drawback to such catalyst systems: namely, the polyethylene products are beholden to the choice of the specific catalyst chosen. To make polyethylene with, for example, a different molecular weight distribution, either very different process conditions can be employed, or a new catalyst system can be explored. When exploring highly variant process conditions, other desirable product attributes may be lost, or the catalyst may not perform at a commercially acceptable level. Alternatively, use of a new catalyst may require reactor shut down.
In view of the forgoing, a need exists for new catalysts systems which when subjected to minor process changes, can make differentiated product with useful properties. The catalyst system recently disclosed in U.S. Pat. Appl. No. 20120252994A1 discloses a system that comprised a bridged metallocene catalyst supported on silica. A specific catalyst, silica supported (CH2)4Si[(CH3)4C5H][C5H5]ZrCl2, was shown to make polyethylene products having different melt flow ratios (MFRs) when the polymerization reactor temperature was changed. In addition, U.S. Pat. Nos. 6,936,675 and 7,179,876 teach that the molecular weight distribution (Mw/Mn) of an ethylene copolymer can be increased by decreasing the polymerization reaction temperature, when using bis-cyclopentadienyl hafnocene catalysts, such as bis(n-propylcyclopentadienyl)hafnium dichloride or bis(n-propylcyclopentadienyl)hafnium difluoride.
We now report a catalyst system which under different polymerization temperatures produces polyethylene with different molecular weight distributions, stress exponents and melt flow ratios in a gas phase reactor. Use of the catalyst system of the present invention, allows one to, for example, tailor the balance between polyethylene processability on the one hand and polymer physical and optical properties on the other hand. Such a method allows one to, for example, tailor the properties of an ethylene copolymer toward a specific end use application.